The Global Positioning System (GPS) consists of 24 earth-orbiting satellites. The GPS satellites broadcast a navigation message via a radio frequency (RF) signal. This signal allows any individual with a GPS receiver to process the GPS signals and determine his or her precise longitude, latitude, altitude, velocity and time anywhere in the world.
Although Global Positioning System (GPS) provides very accurate position and time information, there are times when GPS satellite system malfunctions can introduce errors into the GPS signal transmitted from the GPS satellite. When this occurs, the GPS receivers will not be able to accurately determine position and/or time. Past data has shown that the GPS signal has typically malfunctioned on the average of around 45 minutes a year. When the GPS satellite system is functioning properly and producing accurate GPS data, the GPS data is described as having “integrity.”
GPS errors can be caused by a number of conditions. If one of the GPS satellite transmitter elements or other satellite components fail, the GPS signal waveform can become corrupted. For example, an output amplifier in the GPS satellite may start to malfunction and thereby corrupt the transmitted signal. Another source of error is the failure of the satellite's atomic clock. If a clock failure occurs, the satellite will transmit incorrect time data and introduce error into the computed position information. Another potential error is the transmission of erroneous correction data from GPS ground stations to GPS satellites. GPS ground stations uplink correction data to the GPS satellites every 24 hours. If a ground station sends the wrong correction data, then the GPS satellites will produce inaccurate or erroneous output signals. As an example of this type of error, a ground station could mistakenly send correction data for Tuesday when it was supposed to send correction data for Wednesday.
Errors in GPS signals can lead to severe safety issues or inefficient operation for many systems that use GPS signals such as aircraft systems, transportation systems, weapon systems and so forth. New aircraft navigation systems are being developed which rely on GPS signals for navigation. Errors in the received GPS signal could lead to mid-air crashes. The Federal Aviation Administration (FAA) has a goal of having no more than a 2×10—8 probability of error in the GPS signal without an alert that the signal is hazardous or misleading. With the current GPS system, the probability of error in the GPS signal is on the order of 10—4 per satellite per hour or even higher. Thus, the current GPS satellite could produce a probability of error ten thousand times higher than the FAA's desired goal.
Other proposed systems which utilize GPS signals include intelligent highway systems. These intelligent highway systems use GPS signals to manage traffic by providing autonavigation for the automobiles on the freeways. Similar systems have been proposed for trains. Thus, it will be a very important safety issue for these systems to ensure the integrity of the received GPS signals.
Currently, the Global Positioning System (GPS) system does not have any form of integrity monitoring as part of the system. A system known as the “Wide Area Augmentation System” (WAAS) is currently being designed and developed to provide integrity monitoring of GPS. The WAAS will use a series of new ground stations at known locations all over the world. Each ground station will include a satellite antenna which receives GPS signals from the in-view GPS satellites. Each ground station will use these GPS signals to calculate its own position. By comparing the calculated position with the known position of the ground system, the accuracy and the integrity of the GPS signal can be determined.
If the calculated position is different from the known position, a correction message is generated by the ground station. The ground station transmits the correction message to an independent messaging system such as a geosynchronous satellite. This geosynchronous messaging satellite then broadcasts the correction message to all GPS users in the region. The GPS users then use the correction message to correct the GPS data received from the GPS satellites. Alternatively, the geostationary messaging satellite can transmit an integrity message to all GPS users in the region, informing the users of a potential satellite malfunction. GPS users can thereby be informed that they should not rely on the GPS signals being received. Alternatively, the ground system could send the integrity message to a mission control system which sends a message to the GPS satellites to correct the erroneous data or to cease transmitting all GPS navigation data.
This planned WAAS integrity monitoring system will require an enormous cost including the cost of building the new WAAS ground stations, procuring the new geostationary messaging satellites, and the costs of maintaining and operating the ground stations. Estimated costs for the development and implementation of WAAS are greater than 2 billion dollars. Moreover, the WAAS may not be able to signal a problem with GPS integrity with sufficient speed. Systems using GPS frequently need to know of a change in GPS signal integrity in times less than 1 sec after a malfunction or error occurs.
What is needed is a system that can provide a high level of confidence in GPS integrity without the enormous cost and complexity associated with the WAAS. What is also needed is a system that can correct GPS errors or alert GPS users to a loss of integrity with sufficient speed to satisfy safety concerns and regulatory standards. Lastly, what is needed is a system that can be used either on its own or in conjunction with a WAAS-type monitoring system to provide a high level of confidence in GPS integrity for use in navigation systems, aircraft landing systems, transportation systems, weapon systems, and many other types of systems to provide increased safety and efficiency.